In incremental measuring systems, a scale can be scanned in an optoelectronic or inductive or magnetic or capacitive process, in dependence on the nature of the scale and of the scanning unit. In most cases the scanning results in the generation of basically sinusoidal analog signals, in which a complete signal period is associated with a complete increment of the scale. For an optoelectronic scanning, each complete scale increment comprises a bright field and a dark field and in the simplest case a graduation line and an empty field or free space. In the present case the term "linear incremental measuring system" is used to describe also absolute measuring systems having a plurality of coded scale tracks including an incremental scale track having the highest resolution. In most known linear measuring systems of the present kind, the scanning results in the generation of at least two analog signals which are displaced in phase e.g., by 90.degree., in dependence on the direction of the scanning movement performed by the scanning unit relative to the scale, one of the signals leading the other signal so that the direction of the scanning movement can be detected from the analog signals by a direction discriminator. The analog signals are processed in a processor, which generates digital signals which can be counted. In the simplest case, binary square-wave signals are derived from the zero crossings of the phase-displaced signals by a trigger and are delivered to a bidirectional counter, which is supplied from the direction discriminator with direction-indicating signals indicating the instantaneous direction of the scanning movement and which increases or decreases its count in dependence on that direction. The count of the bidirectional counter indicates the distance of the scanning unit from a zero point, which has been selected or which is defined by a reference mark on the scale. In other embodiments, a separate counter is associated with each direction of the scanning movement and is supplied with those digital signals which indicated a movement in that direction. In the latter case the resulting count is derived from the counts of the two counters by a computation, in most cases by means of a computer. The usual scale divisions and the above-described "direct" processing of the analog signals permit resolution of an order of a hundreth of a millimeter. Such a resolution is the coarsest resolution of the usual linear incremental measuring systm. A number of known methods and circuit arrangements can be adopted to dervie intermediate values from the signals obtained by the measurement in that the increment of an order of one hundreth of a millimeter is electronically subdivided. This may be accomplishedd e.g., by potentiometer circuit and interpolating computers. Such circuits deliver digital signals. In the prior art, a resolution of an order of 1 micrometer is common and a resolution of 0.1 micrometer can be achieved.
In numerous applications of incremental linear measuring systems it is desired to ascertain the instantaneous velocity of the scanning movement in addition to the position of or the distance travelled by the scanning unit, inter alia, in path control systems, machine tools and robots which are provided with incremental linear measuring systems. It is known that the velocity of parts can be ascertained by means of tachometer generators connected to the corresponding drive systems and generating a d.c. voltage, which substantially corresonds to the instantaneous velocity and has a polarity which depends on the sense of rotation. In that case, separate tachometer generators are required for a measurement of velocity. The actual velocity of a member which is driven by a motor to perform a linear movement e.g., a tool slide, must be derived or computed from the information which indicates the rotary speed of the motor. Additional complications are introduced by possibly interposed mechanical transmissions and the slip involved in most cases in the transmission of power from the motor to the driven member, which usually cannot be taken ino account or can be taken into account only in part.
Systems other than tachometer generators can be used to ascertain rotary speeds, angular movements and angular velocities of a motor. For this purpose a magnetic pulse generator connected to the motor comprises a disc carrying a plurality of angularly spaced apart magnets, which are moved past a stationary sensor so that voltages depending on the position of the disc are induced in the sensor. The voltages can be converted to digital signals. In such arrangements the rotary speed, the angular movement and the angular velocity can be digitally indicated or ascertained or an analog display can be obtained in that the digital signals are transformed by a suitable digital-to-analog converter. Such measuring system will have a much coarser resolution than the linear measuring systems of the kind discussed here. Small angular movements and low angular velocities cannot be indicated accurately and such systems also have the basic disavantage that the linear velocity must be derived from the motor speed by a computation and basic errors which are due to slippage or to computation errors may also occur. Moreover, the use of such arrangements in combination with a linear measuring system has not been contemplated before and such a combination would be very expensive.